Micromechanical components, which are used in the automotive sector as acceleration sensors, for example, normally have a microstructure having a movable functional element. The microstructure, which is also called a MEMS structure (micro-electro-mechanical system), may include a rotatable flywheel mass in the form of a rocker, for example. Examples of such sensors are described in European Patent Nos. EP 0 244 581 A1 and EP 0 773 443 B1. The sensors may be read in a capacitive manner, the lever arms of the rocker acting as electrodes and respectively forming a capacitor with two additional counter-electrodes.
For illustration purposes, FIGS. 1 and 2 show a traditional micromechanical component 100 in a schematic lateral sectional view. Component 100 includes a substrate 110 having three planar electrodes 131, 132, 133. A functional layer 150 in the form of a rotatable rocker is provided above electrodes 131, 132, 133, and it has two lever arms 151, 152 having different lengths. A detailed plan view illustration of component 100 is shown in FIG. 3. FIG. 4 additionally shows the lateral placement of electrodes 131, 132, 133 in relation to lever arms 151, 152. A torsion spring is disposed in an area between lever arms 151, 152, which torsion spring has two torsion bars 158 connected to a supporting element 159. Rocker 150 is connected to substrate 110 or a circuit trace level including electrodes 131, 132, 133 via supporting element 159. Due to the different lengths, lever arm 152 has a surface section acting as a supplementary mass 153 in comparison with lever arm 151, so that a mass asymmetry exists in relation to the torsion spring. Furthermore, a hole structure having traversing cut-outs 155 is provided in rocker 150. By this means, an etching medium may be introduced to a sacrificial layer used in the manufacturing of component 100, whereby the sacrificial layer is removed and rocker 150 is exposed (not shown).
The two electrodes 131, 132 respectively form a capacitor with the above-lying lever arms 151, 152, which is used to detect an acceleration in a capacitive manner. Due to the difference in masses of lever arms 151, 152, the influence of an acceleration force F (perpendicular to substrate 110) causes a rotary motion of rocker 150 around an axis of rotation defined by the torsion spring as shown in FIG. 2, which is associated with a distance modification and thus a capacity modification C−ΔC or C+ΔC between lever arms 151, 152 and electrodes 131, 132. Acceleration F may therefore be detected by measuring the capacity or the capacity modification. In this context, electrode 133, which is disposed under supplementary mass 153 of second lever arm 152, is used to block off the influence of an electric potential of substrate 110 on supplementary mass 153 of rocker 150 during operation of component 100, in order to suppress a deflection caused in this manner. To this end, electrode 133 is connected to the same electric potential as rocker 150.
One aspect that is essential for the measuring accuracy of component 100 is the zero point stability, i.e., whether and to what extent the measuring behavior is subject to an offset. In addition to mechanical stress influences, a zero-point offset may be attributed to electrical effects. This includes potential differences between the circuit trace level including electrodes 131, 132, 133, and rocker 150, which are caused by surface charges, for example. The surface charges may be captured in native oxide layers of components of component 100 of silicon and may cause potential differences in a range of several 0.1 V (approx. 100-500 mV). Forces caused in this manner between electrodes 131, 132, which are used for the evaluation, and lever arms 151, 152 of rocker 150 indeed generally operate symmetrically so that no deflection results from this as long as rocker 150 originally stands straight (i.e., is aligned in a manner parallel to substrate 110). However, a potential difference, caused by surface charges, between screening electrode 133 and rocker 150, indicated by voltage U in FIG. 1, has an effective action of force on supplementary mass 153 or lever arm 152 and thus results in a tilting of rocker 150. Since the process-related surface potentials may change with the temperature or during the lifetime of component 100, for example, the tilting of the rocker may change and thus undesired offset signals may result. Such effects pose a large problem in applications for detecting small acceleration values (low-g sensors) such as ESP (electronic stability program), for example, or starting assistance such as HHC (hill hold control).
Furthermore, supplementary mass 153 results in an increased space requirement and also proves unfavorable for the overload resistance of component 100. The different lengths of lever arms 151, 152 result in different maximum acceleration values, as a function of the direction of the acceleration acting on component 100 in a manner perpendicular to the substrate level, starting from which one of lever arms 151, 152 contacts substrate 110 or the electrodes (“impact acceleration”). In the case of the acceleration direction that moves lever arm 152 in the direction of substrate 110 (FIG. 2), the impact acceleration of component 100 is reduced due to supplementary mass 153.